The present invention relates to an improved electronic ballast particularly adapted for use in a fluorescent lamp which is intended to lengthen the operation life of the fluorescent lamp tube and to prevent users from electric shock and also guard the fluorescent lamp and the electronic ballast from damage.
At present, most fluorescent lamps use conventional core ballasts; but several new electronic ballast have been developed which save electricity and can operate without a starter, and work with little interference, and are small in size and light in weight. As a result of the above cited advantages, these novel ballasts are taking the place of conventional ones rapidly. FIG. 1 shows a commercially successful recent electronic ballast. It works rather like a half-bridge converter; the household low frequency AC voltage (60 HZ) is transformed into a stable DC voltage by means of rectifying /filtering circuitry 10A . The converted DC voltage is input to high frequency switching circuitry 20A . After the high frequency switching circuitry 20A is actuated by charging circuitry 30A, i.e., on the charging capacitor being charged by a charging circuitry 30A to such an extent that the DIAC 32A is actuated, a conversion transistor Q1 of the high frequency switching circuitry 20A is first activated to work, and then the driving transformers L1, L2, L3 mounted onto the same core are mutually induced to render the conversion transistors Q1, Q2 to be quickly actuated in turn, i.e. only one of the transistors is turned on at a time, with the other off.
As a result of the high speed interchanged activation of the transistors Q1, Q2, the current in the inductor L4 is directed from left to right on the activation of the transistor Q1 and is directed from right to left on the activation of the transistor Q2. Therefore, the inductor L4 and the lamp tube 40A, received an alternating current. Since the interchanged activation of the transistors Q1, Q2, is rather speedy, the current in the inductor L4 and the lamp tube can be treated as a high frequency alternating current. In other words, when the high frequency switching circuitry 20A has been triggered to act, it can oscillate to output a high frequency alternating current to the lamp tube 40A. The inductor L4, acting as a current ballast, can limit an excessive current from flowing through the lamp tube 40A so as to prevent the lamp tube 40A from being burned by an excessive current. As a result of the supply of this high frequency alternate current, the inductor L4 and a capacitor C1 begin to resonate so as to generate a high resonant voltage at both ends of the lamp tube 40A, causing each filament 41A to be quickly heated with electrons emitted therefrom to activiate the inert gas in the lamp tube to illuminate the lamp. After the lamp tube 40A is actuated, the impedance thereof drops and the current therein is increased so that most of the current will not flow through the capacitor C1, ending up with the inductor L4 and the capacitor C2 resonating. The capacitance of the capacitor C2 is far larger than that of the capacitor C1, and thus the natural resonance frequency is shifted to the lower frequency region. This shift in natural resonance frequency changes the operational Q point of the circuit. As a result, the circuit is not in its optimal resonating state as it is in the starting stage. Therefore, the output voltage and wattage generated by the circuit is substantially lowered after the actuation of the lamp. From then till the cut-off of the electrical power, the lamp tube 40A can be illuminated by way of a steady output voltage and current.
The above cited prior art electronic ballast is characterized in that it is small in size, light in weight, fast to start the lamp, saves in electricity and works without a starter, and the operation frequency is above 25 KHZ so as to produce no flickering; however, there are some disadvantages in practical operation that are as follows:
1. An instantly generated inrush current is applied to the filaments of the lamp tube on the starting of each lamp; as a result the on/off operation on the lamp tube is so frequent that the oxidized substance on the filaments will be dissipated as a result of the high temperature caused by the instant large inrush current after a relatively short period of time, and the filaments are apt to burn out; this will shorten the operation life of the lamp tube greatly.
2. If the lamp tube can not be ignited in a specific time or can not be lighted up at all, the prior art ballast will be constantly subject to a resonating state with continuous high voltages generated at both ends of the lamp tube, causing the lamp tube to be subject to continuous, rather than instant, high voltages resulting in damage to the ballast circuits.
3. As stated in point 2, the abnormal continuous high voltages and power will have a damaging effect on the circuits of the ballast, especially on the conversion transistors which can be easily burned out.
The present inventor has noticed the above cited disadvantages of the hereinbefore described circuit and worked out an improved one which can work better and safer.